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44-                          METALLURGY OF IRON AND STEEL.
of the stock it becomes red hot, whereupon the carbonic acid is expelled, as in an ordinary lime kiln, and the burned lime descends to unite with the silica, which is present in the ore and in the ash of the coke. Without this lime the silicious material would scarcely be fusible, but when the proper quantity is added the lime, silica and earthy.constituents of ore and ash unite to form a fusible slag that flows readily from the cinder notch. The proper proportion of lime-' stone depends upon the impurities in the ore, in the coke and in the stone itself. Some furnaces run on a mixture of ores averaging not over 6 per cent, of silica, while other furnaces average 10 per cent. The stone itself varies in different localities from 1 to 6 per cent, in silica, while the percentage of ash in the coke may be anywhere from 6 to 15 per cent. . A furnace running on silicious ores and limestone and a poor coke will need twice as much limestone as one carrying good ore and fuel, while with such poor material more fuel will be required and twice as much slag produced. An important duty of the lime after it has been fused into slag is to carry away the sulphur in the coke. Much difference of opinion exists as to the proper and possible chemical composition of blast-furnace slags. Koughly, it may be said that the silica should be between 30 and 40 per cent, and the lime between 40 and 50 per cent., and that when.the slag is made more basic the temperature must be raised, as each increase in lime raises the melting point.
SEC. Ilf.—Tlie use of 'burned, lime.—In 100 pounds of pure limestone there are 56 pounds of CaO and 44 pounds of carbonic acid gas (C02). As soon as the stone reaches a red heat in the blast furnace this C02 is driven off and rises through the overlying stock, some of it uniting with the coke according to the following reaction:           •                '                                       ••           ,
This shows that every pound of carbon in the stone carries away a pound of carbon from the coke; that if a thousand pounds of stone be used to one ton of coke, then 6 per cent, of all the fuel is destroyed •by the stone, while if twice that amount of stone be charged, then 12 per cent, is lost. To prevent this waste, some furnaces in Mid-dlesborough, England, as well as elsewhere, have calcined the stone before charging, and there are papers on record showing a very con-